1. Field of the Invention
This invention relates to the field of rotary cutting tools, and in particular to an improved cutting bit arrangement for mounting to a rotary cutting tool which typically carries a number of such cutting bit arrangements on its body.
2. Brief Description of the Prior Art
Rotary cutting tools having a rotating body portion carrying a number of cutting bits are known in the art. Such rotating cutting tools may have as many as 100 or more cutting bits fixed to the body of the rotating cutting tool for cutting, drilling, surfacing, or otherwise forming holes, channels, or tunnels in, hard material or substance formations. Rotary cutting tools of this type are used extensively in the mining industry, in particular.
Replaceable cutting bits for mounting in, and to be retained by, a rotary cutting tool body are also well known. Such cutting bits have a shank portion insertable and locked into a cavity in the rotary tool body and a head portion projecting away from the rotary tool body, a number of separate cutting bits being carried by the rotary tool body. However, each projecting head portion of such a prior art cutting bit typically has a conical nose tip of tungsten carbide brazed on the end of the cutting bit head. The tungsten carbide tip contacts the material being cut in a gouging, scraping, or compression fracturing action, chipping away at the rock or earth strata by the thrust forces applied to the rotary tool body.
In this connection, rock, or stone, may be fractured by means of compressive or tensile forces. It requires much greater compression forces than tensile forces to fracture rock. Prior art rotating cutting tools generally drive their mounted cutting bits into the rock using compressive forces to fracture the rock. With such prior art cutting bits, the compressive fracture forces are directed only outwardly and in the cutting (drilling) direction from the cutter point. As a result, compared to tensile fracturing, greater horsepower is required, wear on equipment and tools is greatly increased, and excessive heat may be generated leading to possible explosion.
There are several other problems associated with such prior art cutting bits. A major problem is wear on the nose tip due to heat and pressure experienced in the cutting process. Such wearing down of the nose tip is not unexpected for a variety of reasons. First, in manufacturing the cutting bit, the tungsten carbide nose piece is the least dense at the extreme end of the conical tip where it is desired to be the most dense. This is due to the axial thickness of the nose piece tip being greatest between the base of the tip and its point. This is a known characteristic of formed tungsten carbide workpieces having regions of varying thicknesses. It would be desirable to have the cutting tip of a cutting bit formed of a hardened material, such as tungsten carbide, having high and constant density throughout.
Secondly, the heat generated during the brazing of a tip to a prior art cutter bit head degrades the hardness of the tip. It would be desirable to attach a cutting tip to a cutter bit without using high temperature techniques.
Thirdly, the same contacting portion of a prior art cutting bit hardened tip is exposed to the severe cutting engagement of the rock or substance being cut, and therefore builds up the temperature at the cutting tip, all contributing to a high wear rate. It would be desirable to provide a cutting bit tip which does not continuously present the same cutting portion of the cutter tip to the substance being cut, thereby allowing cooling of the portion of the cutter tip between cutting engagement strokes.
Additionally, it would be desirable to have the cutter tip effect tensile fracturing and a cutting action on the substance being cut, rather than employing compression fracturing or creating a gouging or scraping action as is made by the prior art conical nose cutting bits.
Prior art cutting bits have no provision to direct chips and debris away from the region being cut, so that debris around the cutting bits builds up and clogs the rotary cutting tool, at which time the rotary cutting tool must be withdrawn and cleaned, leading to inefficient drilling operation and costly down time and associated labor costs.
Another problem associated with prior art cutting bits is the cost of down time and labor involved with removal and replacement of worn or damaged cutting bits. It would be desirable to have a cutting bit with replaceable cutter tips, not practical with brazed-on carbide cone tips of the prior art.
A further problem often encountered with the prior art cutter bits in some applications is the formation of a "bubble" of highly compressed rock dust surrounding the tip of the cutter. Such bubble reduces the cutter efficacy in cutting the substance and creates a type of compression fracturing in the substance being cut rather than the desired tensile fracturing of the substance being cut. Further, it has now been found that the highly compressed dust "bubble" can inhibit the proper cutting operation in certain substances and a structure in the cutting tool is desired for venting such compressed dust and/or preventing a "bubble" from forming.